Researchers at the FAMU-FSU College of Engineering and the National High Magnetic Field Laboratory discovered that atomic-level structural changes occur when the compound tin selenide heats up — changes that help it to conduct electricity but not heat.

The study, funded by the National Science Foundation and Department of Energy, provides information that could lead to new technologies for applications such as refrigeration or waste heat recovery from cars or nuclear power plants. The research was published by Nature Communications.

“Tin selenide is a curious compound,” said Theo Siegrist, a chemical and biomedical engineering professor at the FAMU-FSU College of Engineering. “It has gotten a lot of interest for its special high-temperature thermoelectric properties. Optimizing those characteristics may lead to viable options for sustainable power generation and other uses in the future.”

Scientists already knew that tin selenide had a high thermoelectric coefficient at elevated temperatures, meaning it can create a strong electric current from a temperature gradient. The question was why and how.

The researchers found that as the compound heated up, the bonds between tin and selenium remained mostly unchanged, still connected by three short and several long bonds. But the tin atoms in the compound began to move around, changing from a fully ordered lattice structure into a partially disordered one.

“The initial idea about this change was that the atoms were displaced, but we found that it is an order-disorder phase transition that was actually what was happening,” Siegrist said. “The tin atom was flopping around, so to speak. That was what allowed tin selenide to scatter the energy waves that conduct heat.”

A good thermoelectric material needs strong electrical conductivity but thermal conductivity that is as low as possible. In tin selenide, this is achieved by a dynamic partial disorder of the tin atoms at elevated temperatures that results in a reduction of the heat conductivity.

Siegrist collaborated on the work with researchers from Oak Ridge National Laboratory, or ORNL, and the University of Tennessee, Knoxville. They used a type of particle accelerator at ORNL called a spallation neutron source to test the material. The accelerator shoots protons onto a target to generate bursts of neutrons, allowing scientists to analyze that target’s crystal structure.

By examining what is happening at the atomic scale, researchers can understand what is driving certain properties that engineers may want to optimize.

“This is fundamental research, and we are interested in the mechanism and influence of the material to get it to do what we want in a thermoelectric device,” Siegrist said. “All these ideas can improve energy conversion devices by making them more efficient.”

Along with researchers from ORNL and the University of Tennessee, Simon A. J. Kimber from Burgundy-Franche-Comté University contributed to this paper.

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Researchers at Florida State University and the FAMU-FSU College of Engineering are helping build the solar cells of tomorrow by examining how a next-generation material can operate efficiently under real-world conditions that include baking temperatures and hours of sunlight.

Their work was published in The Journal of Physical Chemistry C.

Most solar cells are made of silicon, but ongoing research is looking at other options, including a material known as perovskite. In addition to acting as a solar cell, perovskites can also enable a phenomenon called upconversion in organic molecules, a process in which the perovskite absorbs low-energy photons and the organic molecules then convert these particles into high-energy photons.

“We wondered if there was another way of using the photons — the particles of energy in light — that otherwise would not be converted into electricity?” said Theo Siegrist, a professor in the Department of Chemical and Biomedical Engineering. “We want to store the energy from one photon until a second comes around and combine the two photons into one that can overcome the barrier.”

“The idea is that one usable high energy light particle is emitted again after the upconversion process,” said Lea Nienhaus, assistant professor in the Department of Chemistry and Biochemistry.

Previous research into perovskites examined how high temperatures and light degrade them but hadn’t considered the upconversion process in perovskite/organic bilayers under real-world conditions. Understanding how these devices work in typical heat and light conditions shows researchers where to direct their efforts for implementation into commercial solar cells.

The researchers placed their upconversion devices on a heating element, raising their temperature to about 60 degrees Celsius. Then they used optical spectroscopy and X-ray crystallography to examine their properties.

They found that the performance of the upconversion device was significantly diminished after exposure to high temperatures, but not because of perovskite degradation. Instead, the organic molecules required for the upconversion process crystallized under the heat, rendering the device ineffective.

“The perovskite on its own, if you heat it and shine light on it, it degrades,” Niehaus said. “As soon as you put the organic molecules on top, it no longer degrades. The organic molecules are contributing to a more long-lived perovskite, which I think is a very useful result. There are still engineering issues to be solved to make perovskite-based upconversion devices viable, but our hope is that this work is part of addressing those issues.”

This study was part of the Journal of Physical Chemistry C’s “Early-Career and Emerging Researchers in Physical Chemistry Volume 2” special issue.

Other co-authors were FSU graduate students Alexander S. Bieber, Colette M. Sullivan, Masoud Mardani; University of Colorado postdoctoral researcher Katherine E. Shulenberger; FSU undergraduate alumnus Gregory Moller; and Argonne National Laboratory researcher Sarah Wieghold.

This research was supported by the National Science Foundation and the U.S. Department of Energy.

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